WASHINGTON, July 7 — A secret military operation in early 2005 to capture senior members of Al Qaeda in Pakistan’s tribal areas was aborted at the last minute after top Bush administration officials decided it was too risky and could jeopardize relations with Pakistan, according to intelligence and military officials.

The target was a meeting of Qaeda leaders that intelligence officials thought included Ayman al-Zawahri, Osama bin Laden’s top deputy and the man believed to run the terrorist group’s operations.

But the mission was called off after Donald H. Rumsfeld, then the defense secretary, rejected an 11th-hour appeal by Porter J. Goss, then the director of the Central Intelligence Agency, officials said. Members of a Navy Seals unit in parachute gear had already boarded C-130 cargo planes in Afghanistan when the mission was canceled, said a former senior intelligence official involved in the planning.

Mr. Rumsfeld decided that the operation, which had ballooned from a small number of military personnel and C.I.A. operatives to several hundred, was cumbersome and put too many American lives at risk, the current and former officials said. He was also concerned that it could cause a rift with Pakistan, an often reluctant ally that has barred the American military from operating in its tribal areas, the officials said.

The decision to halt the planned “snatch and grab” operation frustrated some top intelligence officials and members of the military’s secret Special Operations units, who say the United States missed a significant opportunity to try to capture senior members of Al Qaeda.

Their frustration has only grown over the past two years, they said, as Al Qaeda has improved its abilities to plan global attacks and build new training compounds in Pakistan’s tribal areas, which have become virtual havens for the terrorist network.

In recent months, the White House has become increasingly irritated with Pakistan’s president, Gen. Pervez Musharraf, for his inaction on the growing threat of the Taliban and Al Qaeda.

About a dozen current and former military and intelligence officials were interviewed for this article, all of whom requested anonymity because the planned 2005 mission remained classified.

Spokesmen for the Pentagon, the C.I.A. and the White House declined to comment. It is unclear whether President Bush was informed about the planned operation.

The officials acknowledge that they are not certain that Mr. Zawahri attended the 2005 meeting in North Waziristan, a mountainous province just miles from the Afghan border. But they said that the United States had communications intercepts that tipped them off to the meeting, and that intelligence officials had unusually high confidence that Mr. Zawahri was there.

Months later, in early May 2005, the C.I.A. launched a missile from a remotely piloted Predator drone, killing Haitham al-Yemeni, a senior Qaeda figure whom the C.I.A. had tracked since the meeting.

It has long been known that C.I.A. operatives conduct counterterrorism missions in Pakistan’s tribal areas. Details of the aborted 2005 operation provide a glimpse into the Bush administration’s internal negotiations over whether to take unilateral military action in Pakistan, where General Musharraf’s fragile government is under pressure from dissidents who object to any cooperation with the United States.

Pentagon officials familiar with covert operations said that planners had to consider the political and human risks of undertaking a military campaign in a sovereign country, even in an area like Pakistan’s tribal lands, where the government has only tenuous control. Even with its shortcomings, Pakistan has been a vital American ally since the Sept. 11 attacks, and the militaries of the two countries have close ties.

The Pentagon officials said tension was inherent in any decision to approve such a mission: a smaller military footprint allows a better chance of a mission going undetected, but it also exposes the units to greater risk of being killed or captured.

Officials said one reason Mr. Rumsfeld called off the 2005 operation was that the number of troops involved in the mission had grown to several hundred, including Army Rangers, members of the Navy Seals and C.I.A. operatives, and he determined that the United States could no longer carry out the mission without General Musharraf’s permission. It is unlikely that the Pakistani president would have approved an operation of that size, officials said.

Some outside experts said American counterterrorism operations had been hamstrung because of concerns about General Musharraf’s shaky government.

“The reluctance to take risk or jeopardize our political relationship with Musharraf may well account for the fact that five and half years after 9/11 we are still trying to run bin Laden and Zawahri to ground,” said Bruce Hoffman, a terrorism expert at Georgetown University.

Those political considerations have created resentment among some members of the military’s Special Operations forces.